The Fv1 protein is an endogenous factor in mice that confers resistance to infection by certain classes of murine leukemia virus, a phenomenon referred to as restriction. The mechanism of restriction is not understood, and the low endogenous level of Fv1 in cells has prevented any biochemical or biophysical analysis of the protein. We have now purified recombinant Fv1(n) protein from a baculovirus system and demonstrate that Fv1 exists in a multimeric form. Furthermore, we have mapped the position of two domains within the protein using limited proteolysis. Biophysical characterization of the N-terminal domain reveals that it comprises a highly helical and extended dimeric structure. Based on these biochemical and biophysical data, we propose a model for the arrangement of domains in Fv1 and suggest that dimerization of the N-terminal domain is necessary for Fv1 function to allow the protein to interact with multiple capsid protomers in retroviral cores.